When continuously coating, for example with a layer of metal, elongated metallic strips, for example steel sheet, the strip continuously passes through a bath containing molten metal, usually zinc or aluminium. In the bath, the strip usually passes below a roller immersed in the metal bath and thereafter moves upwards through stabilizing and correcting rollers. The strip emerges from the bath and is transported through a wiping device composed of a set of air-knives, intended to blow off superfluous molten metal from the strip and further back to the bath, to control the thickness of the coating. The gas that is blown out with the knives is usually air or nitrogen gas, but also steam or inert gas may be used. The strip is then transported without support until the coating has been cooled down and solidified. The coated strip is then led or directed via an upper roller for continued processing of the strip, such as, for example, cutting of the strip into separate sheet element or for winding the strip onto a roller. In the normal case, the strip moves in a vertical direction from the roller immersed into the bath through the correcting and stabilizing rollers and the air-knives to the upper roller.
To stabilize the strip, there is used an electromagnetic stabilizing device that is designed to stabilize the position of the strip with respect to a predetermined transport path. The stabilizing device comprises at least a first pair of electromagnetic stabilizing means placed on respective sides of the strip.
When elongated metallic strips, for example steel sheet, are continuously coated with a layer of metal, a uniform and thin coating thickness is aimed at. To achieve this, it is common practice to measure the mass of the coating after the strip has passed through the upper roller, and then to utilize this reading for controlling the air-knives, which are usually located suspended from a beam movably arranged in the vertical direction in a direction towards the strip and arranged so that they may also be angled such that the angle at which the gas hits the coating on the strip may be changed, thus controlling the thickness of the coating.
Due to the geometry of the strip, the distance that the strip must run without any support, its speed and the blowing effect of the air-knives, however, the strip will move or vibrate in a direction that is essentially perpendicular to its direction of transport. It has long been known to deal with this problem of transversal movements by using correcting and stabilizing rollers, a more precise control of the gas flow from the air-knives, and an adjustment of the speed of the strip and/or an adjustment of the distance over which the strip has to run without support. However, if these transversal movements are not reduced, these movements will considerably disturb the exact wiping of the air-knives, which subsequently results in an uneven thickness of the coating which may be visible to the naked eye. At present, it is difficult to establish by means of measurement equipment how uneven the coating thickness is after the strip has passed through the air-knives and the electromagnetic stabilizing device. The difficulty arises from the fact that the measurement occurs slowly, typically around 1-2 Hz, and only at one point of the strip at a time. If the strip then moves at a speed of, for example, 2 m/s, this means that it is not possible to automatically see local defects in the coating thickness or if there is an uneven coating thickness, which manifests itself in the form of stripes on the strip, without using manual/visual inspection of the strip. Horizontal stripes on the strip may have a repetition that corresponds to a frequency of 10 Hz or more, and if there are vertical stripes on the strip these are also very difficult to detect, even if an automatic thickness sensor that traverses is used.
Instead, the control of the coating thickness today occurs by an operator who manually, by visual inspection, controls the strip as the strip passed through the air-knives and the electromagnetic stabilizing equipment a long time ago, and then, if need arises, controls the air-knives and the electromagnets. Because of this late control, there is thus a risk that several meters of the strip are given an inferior surface quality, which means that a considerable amount of the quality of the strip will subsequently have to be downgraded and, in the worst case, be rejected.
Japanese patent specification with publication No. JP-09-202955 shows how the vibrations in a metallic strip may be reduced with the aid of rollers that stabilize and stretch the strip when it has passed the air-knives. The position of the strip in relation to its direction of transport in a plane is measured with a sensor, from where information is forwarded to a computer that carries out a vibration analysis based on the values obtained and together with information about the speed of the strip calculates the optimal tensioning of the strip in order to control the vibrations in the strip.
Japanese patent specification JP 3173755 describes a device for galvanizing a metallic strip, where stabilizing devices are arranged to reduce the vibrations of the strip. The stabilizing devices comprise guide devices arranged at and in contact with the corners of the respective edge of the strip to fix the edges in the desired position as well as an electromagnet arranged in a region opposite to the width of the strip, on opposite sides of the strip and between the respective guide device, to reduce the vibrations of the strip.
One problem with the above-mentioned devices is that they do not provide sufficient stabilization of the strip, and that the devices do not solve the problems of the surface quality of the strip.
In addition, it is known from, for example, U.S. Pat. No. 6,471,153 and WO 2006/101446, to use a plurality of electromagnets, arranged along the width of the strip, which generate magnetic forces acting perpendicular to the strip to damp transversal movements. Sensors are arranged to measure the distance between the strip and the electromagnets and a guide device guides the flow, based on the distance between the strip and the electromagnets that is measured by the sensors, of current through the electromagnets in order thus to damp the transversal movements to ensure that the position of the strip in relation to the predetermined transport path does not deviate too much.
One problem with the above-mentioned solutions is that, due to the fact that the strip vibrates, the coating on the strip, after the strip has passed the air-knives, is uneven and that there is thus a need of cost-effective devices and methods for wiping off and stabilizing elongated metallic strips, where the device provides a significant contribution to the layer thickness becoming more uniform across the whole surface of the strip.